Optical fiber communication is one of main transmission technologies of modern information networks. A photonic integrated circuit (PIC) chip is a core component of an optical communications device in an optical fiber communications system. To eliminate an effect on an optical signal caused by polarization when the optical signal is transmitted in the PIC chip, and ensure transmission quality of the optical signal, the PIC chip needs to separately process optical signals in different polarization states. For example, a polarization beam splitter, a polarization rotator, and the like in the PIC chip may be used to separate a transverse electric wave (TE) mode optical signal and a transverse magnetic wave (TM) mode optical signal for processing in two optical paths. A polarization rotator is an indispensable component of a polarization diversity system.
In the prior art, a schematic structural diagram of a corner-cut type asymmetric polarization rotator is provided. As shown in FIG. 1, the corner-cut type asymmetric polarization rotator includes an input end 11, a corner-cut polarization rotation region 12, and an output end 13. The input end, the corner-cut polarization rotation region, and the output end are connected sequentially. The corner-cut type asymmetric polarization rotator may be made from silicon and is located in a cladding layer whose material is silica. When the TE mode optical signal is input from the input end and transmitted to the corner-cut polarization rotation region, because a rectangular waveguide becomes an asymmetric waveguide with an L-shaped cross-section, mode hybridization occurs on the TE mode optical signal, the TE mode optical signal is converted into a TM mode optical signal at the end of the corner-cut polarization rotation region, and the TM mode optical signal is output from the output end. Alternatively, when the TM mode optical signal is input from the input end and transmitted to the corner-cut polarization rotation region, because a rectangular waveguide becomes an asymmetric waveguide with an L-shaped cross-section, mode hybridization occurs on the TM mode optical signal, the TM mode optical signal is converted into a TE mode optical signal at the end of the corner-cut polarization rotation region, and the TE mode optical signal is output from the output end. In this way, conversion of a polarization optical signal is implemented. However, the polarization rotation region of the corner-cut type asymmetric polarization rotator is implemented by using a partial etching process in a complementary metal oxide semiconductor (CMOS) process, and performance of the corner-cut type asymmetric polarization rotator is sensitive to a process tolerance. The process tolerance leads to uncertainty in etching depth and sidewall angles, and therefore severely affects conversion efficiency.
In the prior art, a schematic structural diagram of a two-layer asymmetric silicon nitride-silicon waveguide polarization rotator is further provided. As shown in FIG. 2, the two-layer asymmetric silicon nitride-silicon waveguide polarization rotator includes an irregular silicon nitride layer waveguide 21 at the first layer and a silicon layer waveguide 22 at the second layer. The first layer is located above the second layer. The silicon layer waveguide at the second layer includes a first isosceles trapezoidal waveguide 221, a rectangular waveguide 222, and a second isosceles trapezoidal waveguide 223. The first isosceles trapezoidal waveguide, the rectangular waveguide, and the second isosceles trapezoidal waveguide are connected sequentially. A vertical spacing between the waveguide at first layer and the waveguide at the second layer is not more than 1 micron. The first isosceles trapezoidal waveguide serves as an input end, the rectangular waveguide and the silicon nitride layer waveguide serve as a polarization rotation region, and the second isosceles trapezoidal waveguide serves as an output end. When a TE mode optical signal is input from the input end to the polarization rotation region, the TE mode optical signal is coupled to the two-layer waveguides in the polarization rotation region, mode hybridization occurs, the TE mode optical signal is converted into a TM mode optical signal at the end of the polarization rotation region, and the TM mode optical signal is output from the output end. Alternatively, when a TM mode optical signal is input from the input end to the polarization rotation region, the TM mode optical signal is coupled to the two-layer waveguides in the polarization rotation region, mode hybridization occurs, the TM mode optical signal is converted into a TE mode optical signal at the end of the polarization rotation region, and the TE mode optical signal is output from the output end. In this way, conversion of a polarization optical signal is implemented. However, the polarization rotation region of the polarization rotator is a linear profile waveguide. In order to implement conversion of the polarization optical signal, the linear profile waveguide in the polarization rotation region needs to be at least 400 microns. Therefore, the polarization rotator is large, which is unfavorable for high-density integration of the PIC chip.
Therefore, how to achieve higher conversion efficiency and ensure a smaller size of the polarization rotator in a process of converting optical signal polarization states is an urgent issue to be addressed.